The long-term objective of this project is to apply the mechanistic insights of the interaction between Rac GTPase of the Rho family and their regulatory proteins, the Dbl family guanine nucleotide exchange factors (GEFs) in particular, to the design of novel approaches to target deregulated Rac activities in human diseases such as cancer. The GEF-Rac signaling axis lies in the crossroads of many signaling events initiated by growth factors, cytokines, stress, and adhesion molecules. Their functional interaction leads to the activation of Rac and a variety of Rac-mediated physiological responses including actin and microtubule cytoskeletal reorganization, adhesion, migration, and proliferation. In the last funding period, we have studied the structure-function relationship of Rac1 in regard of its interaction with GEFs and effectors, and have succeeded in deriving structural and kinetic information of several functional interactions involving Rac1. Further, we have studied the role of Rac1 by a conditional gene targeting approach in mice to reveal several cell functions of Rac1 that could not have been discovered by conventional means. Last, we have discovered a first generation small molecule inhibitor, NSC23766, that is effective in targeting Rac in vitro and in vivo, and have utilized it to study a number of physiological and pathological functions of Rac GTPases ranging from hematopoietic stem cell mobilization, platelet regulation, to schwannoma phenotype reversion. In this proposal, we will (1) pursue structure-function based rational design and improvement of Rac-targeting small molecule inhibitors by screening and medicinal chemistry based on the newly resolved Rac1-NSC23766 crystal structure, and (2) apply the Rac-targeting small molecule inhibitors to leukemia stem cell mobilization from their bone marrow niche in a chronical myeloid leukemia mouse model. These mechanism-based studies of small molecule inhibitor design and pre-clinical validation in a novel pathologic context will not only provide an improved generation of Rac GTPase inhibitors for a wide range of usage in cancer research, but also will implicate an innovative avenue of application in mobilizing leukemia stem cells from their microenvironment, which can be utilized as an important regiment in combinatory therapy for effective eradication of cancer.